In the past various types of retractable trailers have been employed. In general, some of these have employed bell crank axles journalled to the trailer bed which may be operated by a hydraulic cylinder to raise and lower the bed with respect to the wheels. The lowered bed facilitates loading and unloading of the bed while the raised bed provides for proper road clearance in road travel.
Such trailers have required complex linkages for operation in raising and lowering the bed and have been subject to shock and vibration in the raising and lowering of the bed as well as in over the road travel. Further, the provision for raising and lowering tandem wheel sets employing two or three pairs of wheels has occasioned additional stress and complications in the required linkage for operation to the raised and lowered positions as well as road travel.